Ukrainian soldiers sit on their armored personnel carrier as they pass near the eastern Ukrainian city of Donetsk on Aug.11. (ANATOLII STEPANOV/ / AFP/Getty Images)

WASHINGTON — A tense standoff between Ukrainian forces and Russian-backed separatists will be over within a week — and will decide whether “all-out war” begins in Eastern Europe, says a US senator.

Florida Sen. Bill Nelson, the No. 3 Democrat on the Senate Armed Services Committee, leaves for a trip to Ukraine later this week, where he will huddle with leaders of that nation and “representatives” for the top US general in Europe.

Nelson told reporters via a conference call that with Moscow-backed separatists occupying several eastern Ukrainian cities, including Donetsk, “I expect within the matter of another week, this is going to be decided one way or another.

“Either Russia comes across the border or the Ukraine military will have prevailed,” Nelson said.

If the latter occurs, “then it’s just a mop-up operation” for Ukrainian forces, he said.

But if Russian President Vladimir Putin senses his surrogates’ grip on key cities is slipping and decides to invade eastern Ukraine with some or all of the 20,000 troops US officials say are massed on his side of the border, “then it’s Katie bar the door,” Nelson warned.

That scenario could lead to “all-out war,” Nelson said, though he never called for direct American involvement should a shooting conflict between Russia and Ukraine begin.

Nelson did call for the Obama administration to provide Ukraine’s military with materiel to “defend themselves against the Russian bear.” He pointed specifically to things like armored personnel carriers and made a vague reference to “weapons.”

But does Nelson believe a Russia-Ukraine war is imminent?

“I can’t believe Vladimir Putin would be that foolish. There was a time we thought he was coming across the border,” Nelson told reporters. “Instead, he had his ... thugs go in ... to do his work. That hasn’t work. That’s why [several] towns are surrounded by the Ukrainian military.”

The veteran SASC member said all evidence in recent days about the likelihood of war breaking out is “to the contrary.”

“All the events in the last 24 hours indicate they’re about to settle this with the rebels giving up,” Nelson said. “Except if that is unacceptable to the Russian president and he sends his troops across the border.”

Though he never called for America to get involved should war break out, Nelson did say, “American can’t have Russia advance” nor have “Russia violate [Ukraine’s] sovereignty.”

Nelson is worried that other former Soviet bloc nations would be next, including Georgia and several Baltic states.

“Suddenly, Russia is now occupying Soviet states and pressing upon not only Eastern Europe but also Western Europe,” he warned. “So that delicate balance has to be maintained in Europe.

“We need to stand firm,” Nelson said, “and tell Mr. Putin he’s not going to use his aggression and get away with it.”

However, other than sending vehicles and weaponry to Ukrainian forces, he did not specify what steps are needed to accomplish that. ■